Coating of a metallic pigment has been conventionally performed to impart metal texture to a resin molded article such as a thermoplastic resin molded article or the like. Such coating has to be carried out through complex steps including a surface cleaning step, a preliminary coating step and a drying step. An expensive apparatus is required in this coating process, which leads to an increase in process cost. In the event that detective products generated in the coating process and products collected after use are pulverized and recycled, the impact strength of recycled resins is sharply reduced and cannot be reused. Thus the detective products and the used products are disposed as wastes. This poses a problem of increased cost and environmental pollution.
As a solution to this problem, there are available different molding methods that make use of a thermoplastic resin composition containing a metallic pigment. For example, U.S. Pat. No. 6,280,837 discloses a resin molded article formed of a thermoplastic resin composition in which a brightening agent, such as an aluminum powder, a pearly mica pigment or a flaky glass powder, and a coloring agent are directly mixed with a synthetic resin. U.S. Pat. No. 6,143,815 discloses a resin molded product in which a scaly brightening agent having a particle size of 10 to 20 μm in accordance with mixed with a thermoplastic resin. U.S. Pat. No. 5,530,051 discloses a polyester-based resin molded article including metallic micro-plates having an average particle size of 80 to 800 μm and an average shape ratio of 1/100 to 1/8, the metallic micro-plates formed by cutting a metal plate to have a partial cutout portion.
In the prior art cited above, however, the metals used as the metallic pigment are not arranged in a specified direction in an injection molding process but are distributed vertically or obliquely as resin flows join each other. As a result, light is unevenly reflected only in a specific portion. This makes it impossible to obtain a uniform metal color.
In other words, as shown in FIG. 1, if a molded article is produced using a scaly or plate-like metallic pigment, the metallic pigment 100 is not oriented in a specified direction in an injection molding process. In the flow merge line where different resin flows meet each other, the metallic pigment is distributed at an angle perpendicular to or substantially perpendicular to the major surfaces of a molded article. This phenomenon occurs in the portions where the thickness of the molded article is changed. As a consequence, the reflection of light becomes uneven, which makes it impossible to obtain a uniform metal color.
In addition, the metallic pigment 100 protrudes on the product surface, thereby reducing the brightness. A plate-like aluminum pigment protruding on the product surface tends to increase friction on the product surface, consequently reducing the scratch resistance. Moreover, the metallic pigment 100 reduces the stability and durability of the product surface when exposed to a high temperature, a low temperature or ultraviolet rays. This results in a reduced lifespan of the product.